1. Field of the Invention
The present invention relates to a process for making a thermally imageably, negative working printing plate.
2. Brief Description of Art
The use of thermally sensitive, negative working printing plates comprising patterning compositions coated over a hydrophilic underlayer is well known. Several methods for forming images using such compositions are known. All these methods are based on the principle of introducing a differentiation in properties between the image-wise exposed and non-exposed parts of thermally sensitive composition, e.g. difference in solubility, adhesion, tackiness, permeability etc. Such generated difference may be subsequently employed in an additional developing step to produce a visible image. A difference in solubility in an aqueous developer between exposed and non-exposed parts of the thermally sensitive compositions is often used for the production of lithographic printing plates.
The use of the difference in solubility is described in the following documents:
U.S. Pat. No. 4,356,254 (Takahashi et al.) is directed to an image-forming method wherein a light-sensitive material comprising a support having a light-sensitive layer provided on the support. The light-sensitive layer containing a quinonediazide sensitizer. This light-sensitive material is imagewise exposed with a high intensity energy beam (e.g. a laser beam) to make the o-quinonediazide compound alkaline soluble in those areas that are imaged; then overall heating the light-sensitive material to insolubilize the exposed areas; then overall exposing (flood exposing) the thus-processed material with light to solubilize unexposed areas; and then developing with an alkaline developer to provide a negative image by removing alkaline soluble areas of said layer. This reference does not teach employing acid generator-type light sensitive layers.
U.S. Pat. No. 4,356,254 (Stahlhofen et al.) describes a process for producing negative relief copies using a light-sensitive material that contains either a benzoquinone diazide compound or a naphthoquinone diazide compound. This process includes the steps of imagewise exposing the light-sensitive material; then heating the light-sensitive material; then flood exposing the thus processed material which is followed by developing the material, whereby the layer areas which were not struck by light in the first imagewise exposure are washed off. This reference uses quinone diazides as the sensitizers in the light-sensitive material and does not teach the use of acid generator-type sensitizer.
U.S. Pat. No. 4,927,741 (Garth et al.) describes a method for the reversal of photosoluble lithographic printing plates having an aromatic quinone diazide-containing coating. This method includes the steps of (1) imagewise exposing a portion of the coated surface with UV lamp to render that portion soluble in a developer; (2) heating the plate and coating to render the imagewise exposed relatively insoluble in the developer; (3) then overall irradiating the coated surface to solubilize the remaining portion of the coated surface not previously exposed, wherein the overall irradiation step (3) is applied through water. This process does not teach the use of acid generator-type sensitizers, but instead is limited to quinone diazide sensitizers.
U.S. Pat. No. 5,340,699 (Haley) teaches a method of forming a lithographic printing surface comprising the steps of: (a) providing a lithographic printing plate comprising a support and an imaging layer containing an admixture of (1) a resole resin, (2) a novalac resin, (3) a latent Bronsted acid and (4) an infrared absorber; (b) imagewise exposing said lithographic printing plate to activating radiation; (c) heating said lithographic printing plate to provide reduced solubility in exposed areas and increased solubility in unexposed areas; and (d) contacting said lithographic printing plate with an aqueous alkaline developing solution to remove the unexposed areas thereof and thereby form a lithographic printing surface. This process does not teach using a heat treatment between the flood exposure step (c) and the development step (d).
U.S. Pat. No. 5,380,622 (Roser) describes the production of negative relief copies of a recording plate that uses naphthoquinone diazide compounds as the photosensitive compounds. This process includes the step of: (1) imagewise exposing the recording plate using UV, metal halide, xenon or arc lamps; (2) then heating the exposed plate; (3) then uniform exposing the plate to the same light source used in the imagewise exposure; and (4) then developing the thus-processed recording plate with an aqueous alkaline developer to dissolve the alkali-soluble components and form a negative relief copy. This process also does not teach the use of acid generator-type sensitizers, but is limited to quinone diazide sensitizers.
U.S. Pat. No. 5,631,119 (Shinozaki) teaches an image formation process that employs photosensitive composition layer containing a quinone diazide photosensitizer and includes the steps of: (1) exposing the entire surface (flood exposing) of the photosensitive composition layer, to light rays (normally at 290 to 500 manometers) that render the quinone diazide compound soluble in an alkaline developer; (2) then imagewise heating the flood exposed photosensitive composition layer (such as with a thermal head printer); and (3) then developing with an aqueous alkaline solution.
U.S. Pat. No. 5,922,502 (Van Damme et al.) teaches a method for making a lithographic printing plate comprising the steps of (a) imagewise exposing an imaging element having a photosensitive layer and a thermosensitive layer with a laser thereby imagewise rendering the thermosensitive layer transparent to light for which said photosensitive layer has spectral sensitivity; (b) overall exposing the imaged element with light for which said photosensitive layer has spectra sensitivity; and (c) developing the element. This process does not teach an intermediate heat treating step.
European Patent Application EP 0 851 296 A1 (Habenhauer et al.) teaches a process similar to that in U.S. Pat. No. 5,922,502. Again, no intermediate heat treatment is taught.
Japanese Published Patent Application No. 11190902 A2 (Kunio) teaches making a heat mode recording lithographic printing plate having a image forming layer containing a quinone diazide compound and an infrared absorber. This plate is formed by (1) imagewise exposing the image forming layer with laser beams; (2) then flood exposing it to ultraviolet rays; and (3) then developing it with an aqueous alkaline developer.
Japanese Published Patent Application No. 267266 A2 (Mitsumasa) teaches a method for making lithographic printing plates that includes the steps: (1) imagewise exposing a photosensitive layer to visible laser beams; (2) then developing the imagewise exposed photosensitive layer; and (3) then flood exposing the developed photosensitive layer to UV light.
Japanese Published Patent Application No. 089478 A2 (Yasuo) teaches a method for making a photopolymerizable printing plate wherein a photosensitive layer is imagewise exposed to specified exposure light using a laser, then developed and then flood exposed to light at least 100 times the quantity employed in the imagewise exposure.
As illustrated above thermally sensitive compositions can be used in a variety of methods to reproduce images. Among these methods those, which employed patterning compositions containing light-sensitive material that is a mixture of an acid generator, a cross-linking resin or compound, a binder resin and an infrared (IR) absorber, are commonly used for the production of printing plates. The plates are imaged by imagewise exposure to IR radiation to produce imaged areas, which after further processing steps are insoluble in a developer liquid. These parts consist of a coating which results after acid catalyzed network formation between binder resin and crosslinker.
However, these thermal imaging systems require a heat treatment step after IR imaging and before development (commonly referred to as a preheat step) to complete image formation. The window of operation of these compositions was found to be dependent on the amount of heat exposed during both the IR imaging and the preheat step. When inadequate levels of heat were applied to the imaged areas during IR radiation imaging or preheat, then incomplete image formation or weak images may result on the printing press. In contrast, when the heat energy during the preheat step is too high, both imaged and non-imaged parts are hardened. This phenomenon is referred as fogging, the temperature at which that is first observed as first fogging (in following referred as F).
The thermal sensitivity of the resulting printing plate enables them to function, but for some applications, processing a desirable number of plates within a short period of time is problematic. The most important factor for increasing this number is the amount of acid produced during the IR exposure.
The use of high-performance lasers or laser diodes, which employs higher power than previously used imaging equipment and, thus, leading to a shorter exposure time, is sometimes also problematic with some patterning composition layers. When lasers are used as the exposure equipment, portions of the patterning layer may be ablatively imaged, which decreases the amount of coating remaining at the surface after processing, and decreasing the number of copies during print. When laser diode arrays are used as the exposure equipment, then so-called banding can occur indicated by a strip-wise color difference at the surface of processed plates.
Accordingly, there is a need for an improved method that will widen the window of operation of patterning compositions, based on acid generation during imagewise IR exposure, thereby decreasing the required IR exposure energy, improve processability, chemical resistance and the performance of printing plates. The present invention provides an excellent solution to that need.
Therefore, one aspect of the present invention is directed to a process for making thermally imageable negative working compositions comprising the steps of:
(1) providing a patterning composition layer on a substrate, said patterning composition comprising:
(a) at least one thermal-generated acid generator;
(b) at least one cross-linking resin or compound;
(c) at least one binder resin comprising a polymer containing at least one reactive pendent group selected from the group consisting of hydroxyl, carboxylic acid, sulfonamide, alkoxymethylamide and mixtures thereof; and
(d) at least one infrared absorber;
(2) subjecting the patterning composition layer to a two-stage radiation exposure;
(a) one stage being a flood UV-exposure stage; and
(b) the other stage being a imagewise infrared exposure stage;
(3) treating the exposed patterning composition with heat energy; and
(4) developing the heat treated, exposed patterning composition with an aqueous alkaline developer to remove the non-imaged areas of the patterning composition and leaving the imaged areas substantially unaffected.
According to this invention, it has been found that when selected negative working patterning compositions that are both sensitive in the IR and UV wavelengths are treated with both UV flood exposure and the IR imagewise exposure (in either order) prior a preheat step, the speed can be increased and the preheat and banding latitude are widened. A preferred range of UV exposure exists as overexposure would result in too early fogging, a too low working temperature which may result in insufficient cross-linking. The required level of acid can be produced easily and well defined by common UV exposure devices. Advantageously, such devices are installed prior the preheat segment of the processing line for the plate. Such lines are described in U.S. patent application Ser. No. 09/573,126, filed May 17, 2000.
This invention is useful in processing negative working, thermal imaging plates that use patterning compositions containing acid generator and cross-linked resin precursors. One example of such patterning composition is DITP plates commercially available from Kodak Polychrome Graphics LLC.
This invention also increases the preheat and banding latitude and increases the IR sensitivity of the plate, thereby also reducing ablation.
FIG. 1 is a graph of the measured processing conditions shown in Example 2 with a working temperature fogging pointxe2x88x925xc2x0 C. (Fxe2x88x925).
FIG. 2 is a graph of the measured processing conditions shown in Example 2 with a working temperature fogging pointxe2x88x9210xc2x0 C. (Fxe2x88x9210).
FIG. 3 is a graph of the measured processing conditions shown in Example 2 with a working temperature fogging pointxe2x88x9215xc2x0 C. (Fxe2x88x9215).
FIG. 4 is a graph measured densities for the processing conditions in Example 3.
FIG. 5 is a graph of the measured densities for the processing conditions in the Comparison Example where the UV flood exposure occurs after the preheat step.